1. Field of the Invention
The present invention relates to a load cell, and particularly to a load cell provided with a strain body and a sensor for detecting a displacement of the strain body caused by a load.
2. Background Information
In general, a load cell used in a measuring device or the like has a strain body (i.e., a body to be strained) having a movable portion and a fixed portion, and a sensor for detecting a displacement of a strain body caused by a load applied thereto. A result of the detection is converted into a weight of a measuring target object, i.e., an object to be measured.
The movable portion of the strain body of the load cell is fixed to a support member supporting the measuring target object, or is fixed to a connection member, which connects the support member to the strain body, and will be referred to as a xe2x80x9cmeasuring target side memberxe2x80x9d hereinafter. The fixed portion of the strain body is fixed to a fixed base or a connection member, which connects the fixed base to the fixed portion of the strain body, and will be referred to as a xe2x80x9cfixed base side memberxe2x80x9d hereinafter. When a load of the measuring target member acts on the strain body via the measuring target side member, the movable portion of the strain body is displaced downward relatively to the fixed portion. This displacement is detected by a sensor such as a strain gauge, and is converted into a weight.
The strain body of the load cell is often made of aluminum or aluminum alloy primarily in view of demand for high precision and workability. The measuring target side member and the fixed base side member are primarily made of a material such as steel or stainless steel.
Accordingly, a connection between the strain body and the measuring target side member as well as a connection between the strain body and the fixed base side member are in such a state that dissimilar metal materials are electrically in contact with each other. Therefore, if an atmosphere containing oxygen and/or moisture is present around these connections, electricity flows between the dissimilar metal materials to cause galvanic corrosion so that corrosion expands rapidly to reduce a lifetime of the load cell.
For overcoming the above disadvantages, the strain body may be made of stainless steel. However, it is difficult to ensure a high weight detection precision, and particularly, it is difficult to use the stainless steel instead of aluminum from the viewpoint of a processing or working cost.
For preventing the galvanic corrosion, the strain body may be coated with a molded material such as silicone or polyurethane. However, the molded material, which is present between the member on the measuring target side or fixed base side and the strain body, deteriorates a weight detection precision due to viscoelastic properties of the molded material. If a molded material having a thickness from hundreds of micrometers to several millimeters is present between the member on the measuring target side or fixed base side and the strain body, this lowers the degree of fixing between the measuring target side member and the strain body as well as the degree of fixing between the fixed base side member and the strain body. Also, deformation of the molded material caused by the load deteriorates the weight detection precision. Accordingly, molding is effected on the strain body, which is already attached to the measuring target side member and the fixed base side member. However, this requires a complicated work for molding, and increases a working cost.
The molding may be effected on the strain body alone while masking the movable portion and fixed portion. However, this cannot prevent the galvanic corrosion.
Further, for preventing the corrosion of the strain body, the strain body may be covered with a box made of stainless steel after attaching the measuring target side member and the fixed base side member to the strain body. However, this requires a devised structure for preventing restraint on movements of the measuring target side member and the movable portion of the strain body, resulting in disadvantages relating to the cost and size.
As another manner for preventing the galvanic corrosion, a film having an insulating property may be interposed between dissimilar metal materials. In this manner, however, water enters a fine space between the strain body and the film to cause corrosion. As still another manner, a member made of a relatively soft material may be interposed between dissimilar metal materials. In this manner, however, the load of the measuring target object is not transmitted 100% to the load cell so that an error occurs in a result of measurement.
If a scale including the strain body is used, e.g., in hot and humid surroundings or in a place exposed to salt-laden moisture of seawater or the like, corrosion occurs on the contact surfaces of dissimilar metal materials, and is also liable to occur on other surfaces.
In view of the above, there exists a need for a load cell which overcomes the above mentioned problems in the prior art. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.
An object of the invention is to provide a load cell, which can suppress galvanic corrosion in a contact portion between the strain body and the member attached or connected to the strain body, and also can ensure a sufficient degree of fixing between the strain body and the member attached to the strain body so that a high weight detection precision can be achieved.
Also an object of the invention is to provide a load cell, which has improved resistance to water, rust and corrosion, and can achieve high weight detection precision.
According to a first aspect, a load cell includes a strain body, a bridge circuit and a coating film. The bridge circuit is formed of a strain gauge arranged on the strain body. The coating film is formed on the strain body, covers at least a portion of the strain body attached to a member made of a metal material dissimilar to the strain body, includes a coating layer made of resin having a glass transition temperature of 40xc2x0 C. or more, and has an electrical insulating property.
According to the load cell, when a load on a measuring target side acts on the strain body, a strain gauge detects a displacement of the strain body caused by the load. Since the degree of displacement of the strain body depends on the magnitude of the load applied by a measuring target object, the weight of the measuring target object is obtained from the result of detection of the strain gauge.
In the above structure, the coating film having the insulating property covers the portion of the strain body, to which the dissimilar metal member (i.e., the member made of a metal material dissimilar to the strain body) is attached, so as to prevent galvanic corrosion in such a case that the dissimilar metal member is in contact with the strain body. Thus, in the structure having the dissimilar metal member attached to the strain body, the coating film having the insulating property is interposed between the dissimilar metal member and the strain body. Therefore, an electricity hardly flows between the dissimilar metal member and the strain body, although these are made of different kinds of metal, respectively.
The dissimilar metal member attached to the strain body may be a fixing member for fixing a portion of the strain body to the fixed base, a movable member for transmitting a load of the measuring target object to another portion of the strain body, a stop or limiter member such as a screw or a metal piece for restricting deformation of the strain body caused by an excessive load, and/or a board provided, e.g., with a line terminal portion between the strain gauge and external wiring and/or an amplifier for amplifying a strain gauge output.
In contrast to the above, if a coating film were not present between the dissimilar metal member and the strain body, the strain body would be corroded due to a current flow between the dissimilar metal member and the strain body when oxygen and moisture were present therebetween.
According to the load cell of the first aspect, as described above, the coating film covers at least the portion of the strain body, which may be in contact with the dissimilar metal member. Therefore, the strain body is not in direct contact with the dissimilar metal member attached thereto, and is in contact with the same through the coating film. Since the coating film has the insulating property, a current does not flow between the dissimilar metal member and the strain body, and thus the flow between dissimilar metal materials is interrupted.
In the above structure, since the coating film is interposed between the strain body and the dissimilar metal member attached thereto the degree of fixing between the strain body and the dissimilar metal member may be lowered, and the weight detection precision may be adversely affected. However, these disadvantages are extremely smaller than the disadvantages caused by a thick molded member or other thick members for the insulation. Also, the glass transition temperature is 40xc2x0 C. or more, and therefore the coating film is hard in a temperature range from xe2x88x9210xc2x0 C. to 40xc2x0 C., which is an operation temperature range of this kind of scale. In other words, since the coating film is extremely thin and hard, high weight detection precision can be maintained while insulating the strain body from the dissimilar metal member fixed or attached thereto and hardly affecting the degree of fixing of them.
The strain gauge may be attached directly to the strain body, or may be formed on the strain body as disclosed in Japanese Patent No. 2,506,064.
The coating film may be formed by application or by another manner such as vapor deposition or sputtering.
The coating film of the load cell of the invention may be formed not only on the portion of the strain body to be in contact with the dissimilar metal, or may be formed of the whole surface of the strain body, as will be described later. In this case, the surface other than the surface in contact with the dissimilar metal may be coated with a film of the same material as that covering the dissimilar metal contact surface or a film of a different material. The portion other than the dissimilar metal contact portion may be molded with a conventional mold material.
The coating film may be organic paint, or may be organic/inorganic hybrid paint (ceramic paint), as will be described later.
For example, acryl resin filled with an inorganic filler may be used for the dissimilar metal contact surface, and a conventional rubber-contained resin may be used for the other surface.
In the case where the coating film layer is formed over the whole surface of the strain body, it is possible to interrupt the current between the strain body and the dissimilar metal member, and thereby to suppress corrosion of the strain body. Further, it is possible to suppress contact of oxygen and moisture in an atmosphere with the strain body, and thereby to suppress occurrence and progression of corrosion of the strain body. In this case, therefore, the resistance to water, rust and corrosion can be improved.
In conventional structures, an elastic material such as rubber or polyurethane has been used in a film, which coats the strain body for achieving intended resistance to water, rust and galvanic corrosion, in view of suppressing by the elasticity an influence on deformation of the strain body.
A scale (measuring device) provided with a load cell of the same type as that of the present invention is generally used in a temperature range (i.e., operation temperature range) from xe2x88x9210xc2x0 C. to 40xc2x0 C., and the elastic properties and viscosity properties of the foregoing material such as rubber or polyurethane have a temperature dependency in this operation temperature range. Particularly, if the weighing capacity is small (10 kg or lower), the viscosity properties exert a large influence, which cannot be substantially corrected.
In this load cell, therefore, the coating layer is formed by coating the strain body surface with resin having a high glass transition temperature, i.e., with hard resin so as to suppress an adverse effect, which may be exerted on the change in electric resistance of the strain gauge by the load. Since the change in viscosity properties of this kind of resin is extremely small even in the above operation temperature range, it is possible to provide the load cell having high precision and good resistance to water.
As already described, if a conventional structure is provided with an insulating film between the surfaces of the strain body and the dissimilar metal member, water enters a space to cause corrosion. If a soft material is interposed, a load of the measuring target object is not accurately transmitted to the load cell, resulting in a measuring error.
According to the load cell of the above aspect, however, the strain body surface is coated with the resin having a high glass transition temperature, and thus the hard resin having an insulating property as described above. Therefore, no gap is present between the strain body and the coating film, and thus entry of water does not occur so that the load cell can have a good resistance to water.
Since the hard resin is interposed between the dissimilar metal materials, the load applied from the measuring target object is reliably transmitted.
According to a second aspect, a load cell includes a strain body, a bridge circuit and a coating film. The bridge circuit is formed of a strain gauge arranged on the strain body. The coating film is formed on the strain body, covers at least a strain occurring portion of the strain body except for a portion attached to the strain gauge, includes a coating layer made of resin having a glass transition temperature of 40xc2x0 C. or more, and has an electrical insulating property.
In the structure having the coating film interposed between the strain body and the strain gauge, a displacement of the strain body cannot be accurately reflected in an output value of the strain gauge in some cases, depending on the properties and thickness of the coating film. In the load cell of this aspect, therefore, the coating film is not formed over the portion, to which the strain gauge is attached. For this, such a manner may be employed that the paint is applied to the strain body while masking the portion to be used for attaching the strain gauge.
In the structure having wiring connected to the strain gauge, it is preferable to cover the wiring and the strain gauge with waterproof resin. In this case, since the strain gauge and the wiring are covered with the waterproof resin covering at least the masked portion described before, such a situation can be suppressed that moisture in an atmosphere adversely affects the strain gauge and the wiring. Since the strain gauge is covered with the resin, the surface of the strain body (i.e., uncoated surface) is not exposed even at the vicinity of the strain gauge. This can further suppress occurrence and progression of corrosion of the strain body.
The strain gauge and the wiring may be covered, e.g., by applying silicone resin over a waterproof butyl rubber sheet.
According to a third aspect, a load cell includes a strain body, a bridge circuit and a coating film. The bridge circuit is formed of a strain gauge arranged on the strain body. The coating film is formed on the strain body, covers at least a portion of the strain body attached to a member made of a metal material dissimilar to the strain body and a strain occurring portion of the strain body except for a portion to be attached to the strain gauge, includes a coating layer made of resin having a glass transition temperature of 40xc2x0 C. or more, and has an electrical insulating property.
This aspect particularly relates to a structure, in which the coating film is formed at least over the dissimilar metal contact surface and the strain occurring portion other than the portion, to which the strain gauge is attached.
According to a fourth aspect, the load cell of the third aspect further has such a feature that the resin of the coating layer is acryl resin.
More specifically, the foregoing resin having a high glass transition temperature may be acryl resin (Tg=90xc2x0 C.-105xc2x0 C.), melamine resin (thermosetting resin), phenylic resin (thermosetting resin) or fluororesin (PTFE: polytetrafluoroethylene resin, Tg=130xc2x0 C.; PFA: perfluoroalkoxyethylene, Tg=75xc2x0 C.). The acryl resin is superior because it allows easy introduction of cross linkage as well as easy production of hybrid composition with inorganic filler, and also allows easy application for coating.
In the load cell of this aspect, therefore, the acryl resin is used for coating the strain body so that intended precision of the scale can be maintained without preventing galvanic corrosion, and further the strain body may have improved resistance to water.
The acryl resin may be the same as that disclosed in pamphlet No. WO96/34063. It is preferable to use (meth-)acrylate of lower alkyl alcohol, and is particularly preferable to use copolymer of methyl methacrylate and hydroxyethyl methacrylate.
According to a fifth aspect, the load cell of the fourth aspect further has such a feature that the acryl resin is cross-linked to form a three-dimensional structure.
The resin having the three-dimensional structure, which is produced by the cross-linking, is superior in stability of the coating layer against changes in environment, and particularly in resistance to water and moisture. In this aspect, therefore, the strain body is coated with such resin to provide the coating film having good resistance to water and moisture so that the strain body can have improved resistance to water.
In this aspect, the resin having a high cross-link density is used so that the coating layer can be hard and resistant to a scratch or damage by an external force. Therefore, the strain body can keep intended resistance to rust for a long term.
For introducing the cross-links in the resin, it is preferable to employ such a manner that a monomer (e.g., hydroxyethyl methacrylate) having a functional group capable of forming cross-links is partially copolymerized, a di-isocyanate compound is mixed in the resin, and a heat treatment is effected on the coating layer applied to the strain body. Thereby, it is possible to provide the three-dimensional structure by introducing the cross-links.
According to a sixth aspect, the load cell of the fifth aspect further has such a feature that the coating film further has an anodized aluminum layer, and the coating layer is formed on a surface of the anodized aluminum layer.
The strain body is preferably made of aluminum in view of weight precision. If the strain body of aluminum is anodized, a corrosion-resistant oxide coating is formed over the surface of the aluminum, and many fine holes are formed at the surface of the strain body. These holes improve the adhesion of the coating layer to the strain body. Even when a damage is formed on the coating layer, the background layer formed of the hard anodized aluminum layer can prevent exposure of the aluminum alloy having low corrosion resistance. The anodizing in synergy with the application of the coating film effectively suppresses the galvanic corrosion, and can provide the rustproof performance.
By forming the coating film after the anodizing of the strain body surface, the resistance to acid and alkali, which cannot be sufficiently achieved by the anodizing alone, can be improved.
Further, the electrical insulating property of the anodized aluminum layer improves the electrical insulating property in the whole coating film.
Copper-contained aluminum alloy, which is generally used in load cells, cannot have sufficient resistance to galvanic corrosion even if anodizing is effected on the surface because the anodized aluminum film thus formed have many defects. According to the load cell of the present invention, however, the coating layer is additionally formed on the surface of the anodized aluminum layer so that the coating layer fills the defects in the anodized aluminum film.
According to a seventh aspect, the load cell of the sixth aspect further has such a feature that the coating film has a thickness equal to or larger than 1 xcexcm and smaller than 100 xcexcm.
In this aspect, the coating film has the thickness not exceeding 100 xcexcm so that an intended weight detection precision may be maintained by ensuring an intended degree of fixing between the strain body and the dissimilar metal member attached to the strain body. Thereby, reduction in the degree of fixing, which may be caused by the interposition of the coating film between the strain body and the dissimilar metal member, can be suppressed to an extent, which hardly affects the weight detection precision.
In a conventional structure including a molded material, which has a thickness from hundreds of micrometers to several millimeters, and is present between the strain body and the dissimilar metal member, it is probably impossible to maintain the intended weight detection precision. According to the present invention, however, the insulation is achieved by the coating film smaller than 100 xcexcm in thickness so that the galvanic corrosion can be prevented while maintaining the weight detection precision.
The appropriate thickness of the coating film depends on the kind and size of the load cell. If the thickness of the coating film were excessively small, and did not exceed one xcexcm, a damage would often occur on the coating film so that the load cell could not have intended water resistance. If the thickness were larger than 100 xcexcm, the detection precision of the sensor would be excessively low. Accordingly, the desired thickness is in a range from hundreds of micrometers to several millimeters. It is also preferable that the coating film has a uniform thickness.
According to an eighth aspect, the load cell of the third aspect further has such a feature that inorganic filler having a particle diameter of 50 xcexcm or less is mixed in the resin of the coating layer.
The resin containing the inorganic filler is further preferable because it has resistance to smearing in addition to a sufficient hardness. In this aspect, the above resin covers the strain body for maintaining the precision of the scale while preventing the galvanic corrosion.
The inorganic filler is preferably formed of fine particles of aluminum oxide, silicon oxide or the like. It is preferable that the inorganic filler is uniformly dispersed in the resin. For this, the fine filler having a particle diameter of 50 xcexcm is used, and is mixed in the resin at a rate from about 5% to about 60%.
In a ninth aspect of the invention, the load cell of the eighth aspect further has such a feature that the resin is acryl resin cross-linked to form a three-dimensional structure.
This load cell can achieve operation and effect similar to those of the load cell of the fifth aspect.
In a tenth aspect, the load cell of the ninth aspect further has such a feature that the coating film further has an anodized aluminum layer, and a coating layer is formed on a surface of the anodized aluminum layer.
This load cell can achieve operation and effect similar to those of the load cell of the sixth aspect.
According to an eleventh aspect, the load cell of the tenth aspect further has such a feature that the coating film has a thickness equal to or larger than 1 xcexcm and smaller than 100 xcexcm.
This load cell can achieve operation and effect similar to those of the load cell of the seventh aspect.
According to a twelfth aspect, the load cell of the third aspect further has such a feature that the resin of the coating layer contains inorganic filler having a particle diameter of 50 xcexcm or less, and the inorganic filler is combined with the resin by chemical coupling.
Ceramic paint (organic/inorganic hybrid paint) is produced by combining organic and inorganic components by chemical coupling, which is introduced between the different components. As a result, the ceramic paint has the features of both the components. In the ceramic paint, the density of cross-links between organic polymers is high, and the density of cross-links between organic and inorganic polymers is also high so that the coating film has a dense structure, and thus has high resistance to moisture and water. Accordingly, the coating layer made of such paint can improve the waterproof and rustproof performances.
Since the ceramic paint has a high cross-link density, the coating layer made of the ceramic paint is hard, and is sufficiently resistant to a scratch or damage, which may be caused by an external force. Further, the inorganic components of the ceramic paint, which have resistance to smearing, weather and chemical attack, can further improve the rustproof performance of the coating layer.
In this aspect, therefore, the coating layer is made of the ceramic paint so as to provide the surface of the strain body having appropriate hardness, smear resistance (affinity for water), electrical insulating property and others.
According to a thirteenth aspect, the load cell of the twelfth aspect further has such a feature that a silane treatment is effected on a surface of the inorganic filler.
If the filler in the resin is chemically coupled with the resin to form composites, it promotes formation of the three-dimensional structure of the resin, and contributes to formation of a hard and stable coating film.
In this aspect, therefore, predetermined processing, and particularly, the silane processing is effected on the surface of inorganic filler so as to achieve chemical coupling with the resin. Thereby, a functional group such as an amino or hydroxyl group is introduced into the filler surface. Further, a heat treatment is effected after the application of the resin so that the chemical coupling between the inorganic filler and the resin can be promoted via a multifunctional (usually, bifunctional) compound (crosslinking agent) in the resin.
In this case, the heat treatment is substantially performed at a temperature from 150 to 200xc2x0 C. for 10 to 30 minutes. Application of the resin is preferably performed by a spray, a brush or the like.
According to a fourteenth aspect, the load cell of the thirteenth aspect further has such a feature that the resin is an acryl resin cross-linked to have a three-dimensional structure.
This aspect relates to a case where the coating layer employs ceramic paint (organic/inorganic hybrid paint) made of the organic component of the acryl resin and the inorganic component of the inorganic filler.
As already described, the coating layer made of the organic/inorganic hybrid paint has an improved hardness. In the load cell of this aspect, therefore, the strain body can have improved resistance to water, rust and corrosion.
According to a fifteenth aspect, the load cell of the fourteenth aspect further has such a feature that the coating film further has an anodized aluminum layer, and the coating layer is formed over the surface of the anodized aluminum layer.
The load cell of this aspect can achieve operation and effect similar to those of the load cells of the sixth and tenth aspects.
According to a sixteenth aspect, the load cell of the fifteenth aspect further has such a feature that the coating film has a thickness equal to or larger than 1 xcexcm and smaller than 100 xcexcm.
The load cell of this aspect can achieve operation and effect similar to those of the load cells of the seventh and eleventh aspects.
These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.